Best-Case Scenario

Star tracks lead astronomers to an inescapable conclusion about an inescapable object at the heart of the Milky Way galaxy

From a planet at the center of the Milky Way galaxy, stars would blaze across the heavens like the lights of a sky-spanning Broadway marquee — a million times more stars than we see from Earth, with many shining as brightly as the Moon. Their configurations would change from year to year as they raced around the galactic center. And amid the stellar chaos, a tiny blue dot would glow faintly, almost invisibly: a thin cloud of gas around a supermassive black hole — the galaxy's dark heart.

This panorama of the galactic center was compiled from infrared and X-ray images by space telescopes. The supermassive black hole and its surrounding star cluster form the bright clump at lower right. [NASA/ESA/SSC/CXC/STScI]

The black hole itself is invisible to us. Like all black holes, it emits no energy of any kind, so it's completely dark. And it is so small and far away that not even the most powerful telescopes can see its silhouette against the background of more-distant stars — yet.

Zeroing in on the Black Hole
Your browser does not suppport the HTML5 audio element.Researchers discuss the techniques used to measure the mass of the galaxy's supermassive black hole.

Not-So-Active HeartYour browser does not suppport the HTML5 audio element.UCLA researchers Andrea Ghez and Mark Morris discuss why the Milky Way's central black hole is much quieter than many others.

Astronomers know quite a bit about the black hole, though. It's about four million times the mass of the Sun. It's surrounded by a cluster of young stars, some of which plunge to within a few billion miles of the black hole. And although it's quiet today, a century ago it gorged on a clump of matter that passed too close, creating a pyrotechnic display that lit up the Milky Way's heart.

"This is the best case we have for a supermassive black hole anywhere in our universe," says Andrea Ghez, a professor of physics and astronomy at UCLA and one of the leading experts on the Milky Way's center. "What's happening in other galaxies made us ask the question in the first place, but in fact we have the best evidence for the existence of these incredibly exotic objects from the center of the galaxy."

Much of that evidence has been amassed by two teams, one led by Ghez and another by Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Germany. Using giant telescopes in Hawaii and Chile and cutting-edge observational techniques, they have probed closer to the black hole than ever before. That has allowed them to plot the orbits of giant stars that pass hair-raisingly close to the black hole, providing the best measurement of the black hole's mass. And it has revealed that some of the stars around the black hole are far younger than expected.

"There's been a myriad of surprises, which is the fun of doing research," says Ghez. "You go in expecting to answer one thing and you come out the other end with more questions than you started with."